In the prior art relating to the present invention there exists a considerable number of flat panel display devices which utilize the selective ignition of a gaseous medium for creating a visual display, and which are constructed of a plurality of gas cells, which are defined either physically or electrically as being regions wherein a controlled ignition may be perpetuated. An entire display screen or panel may therefore be created by the interconnection of a matrix of such gas cells wherein a display image may be created by electrically energizing combinations of electrodes protruding from the panel edges and wherein an image may be sustained on the screen by periodically electrically pulsing certain of said electrodes so as to renew cell ignitions.
The present invention is particularly adaptable to gas display panels wherein the display image is positioned on the panel by means of a time sequencing matrix of electrical signals impressed upon the panel electrodes. In this type of matrix, a set of ignition electrodes are generally wired along one edge of the panel and are adapted to receive data signals representative of the image to be displayed. The data impressed upon these electrodes is time-sequenced in combination with timing signals impressed on wires projecting along a perpendicular panel edge, which has the desired result of shifting a cell ignition region serially along a gas channel according to the timing sequence applied. Such a panel has the inherent disadvantage that a fullscreen image cannot be immediately displayed on the panel, but must be time-shifted according to the design parameters associated with the panel. On the other hand, this type of panel has the distinct advantage of minimal electronic circuitry and simplicity of design of the display panel gas cells. By using time-sequencing of the electrical lines coupling the gas channels, there is no need to provide electronic addressing capability to properly position data on the screen, but rather all that is needed is to monitor a counting mechanism to determine when the data has moved to the appropriate time-related physical position on the screen. Furthermore, since the time-shifting of data across a screen of this type can readily be accomplished at speeds far in excess of anything capable of observation by the human eye, it is an adequate approach to data display. The display unit is intended for visual observation by an operator, and under most typical operating situations the operator is unable to even detect that the data he is observing on the screen has been shifted across the screen to its apparent fixed position.
The time-sequenced voltages applied for shifting visual information in prior art devices have comprised bi-level or tri-level voltages which are used for the dual function of initially moving visual information to its proper and appropriate position on the display screen and thereafter for periodically reenergizing the selected spots to maintain the image on the screen. Frequently, prior art devices utilized a first periodic voltage sequence for sustaining the image on the screen and second and third sequenced voltage levels for initially energizing and shifting data along the screen. Since the inherent physics related to any gaseous cell demand that an ignition voltage be sufficiently high to exceed the ionization potential of the gas being ignited, and the sustaining voltage necessary to maintain ignition is a predetermined lesser value, it was frequently assumed to be necessary to provide at least two levels of driving voltage in addition to the zero or ground voltage level. These multiple voltage levels were achieved by either switching different voltage sources selectively into the system or by super-imposing an incremental firing voltage atop a predetermined sustaining voltage at appropriate times.
The specifications of the patent applications incorporated herein by reference disclose an apparatus wherein only a single voltage other than ground potential is needed for both initial cell ignition and sustaining ignition once it has been started. A unique pilot cell geometry enables the accomplishment of this result, together with the selective and sequential activation of lines adjacent one another in the panel. However, since the relative ease or difficulty with which a given cell in a gas channel may be ignited is somewhat dependent upon the history of that cell's ignition, as well as the history of ignition of adjacent cells, the voltage margins required to ensure a repeatable and reliable ignition heretofore were excessively high. These voltage margin requirements dictated the design of voltage generators which are operated above the worst-case cell voltage ignition range to insure that any cell history and electrode configuration would result in ignition. However, overdriving cells which characteristically ignite at a lower firing voltage tends to cause adjacent cells to ignite and thereby destroys the display reliability and repeatability. Consequently, display panels designed according to the teachings of the prior art must be carefully "tuned" to just the right voltage driving levels in an attempt to find an operating voltage range which is suitable for all physical and electrical cell configurations. Since this is difficult in the general case, particularly under normal manufacturing tolerance conditions, the net result is a high rate of reject panels in the manufacturing process which consequently increases the relative cost of those panels which are selected and tested to meet the permissible voltage operating margins. It would be an apparent advance in the art to discover product control techniques which would permit the voltage margins to be loosened without subsequent reduction in product reliability. Alternatively, it would be a significant advance in the art if looser voltage margins could be used in a scheme which permitted such margins by merely selecting an optimum and preferred timing cycle.
Accordingly, it is the principal object of this invention to permit the reliable and repeatable operation of a display panel, of the type disclosed in the aforementioned cross-reference and herein, solely by the sequential timing of voltage signals to be applied to the panel. It is a further object of the invention to provide such reliable and repeatable display circumstances without necessitating extremely close design control on voltage margins. This is a further object of this invention to provide such reliability and repeatable operation in the display panel without increasing the manufacturing cost of said panel.